Linux command lines I always forget!

Various list commands:
Normal - ls
Display dir contents (long format) - ls -l
Display dir contents (reverse) - ls -r
Display dir contents (hidden files) - ls -a

Directory creation:
mkdir folder1

Delete directory:
rmdir folder1

Delete directory and its contents:
rmdir -r folder1

Copy directories:
cp -r folder1 folder2

Move directory:
mv folder1 folder2

Delete file or directory:
rm /home/cheese.doc

View a file:
view xx (q to quit)

Show disk space:
df (see other posts on df)

Show all running processes:
ps aux

Copy a file to  make a backup:
cp /etc/X11/xorg.conf /etc/X11/xorgbak.conf

Restore the backup:
cp /etc/X11/xorgbak.conf /etc/X11/xorg.conf

Keep an eye on any hardware you plug into your Linux box
dmesg | tail -f

Modify the EXIM4 MTA on Ubuntu to mail to the outside world:
dpkg-reconfigure exim4-config